Footage of Anthony Baxter's forcible arrest with his colleague Richard Phinney in July last year became a pivotal part of their documentary You've Been Trumped, which has gone on to win several awards and had sell-out audiences at screenings.

Backed by the National Union of Journalists, Baxter complained to Grampian police about his detention as he interviewed a neighbour of Trump's, which lead to him being handcuffed, fingerprinted, having DNA samples taken and his equipment impounded.

Baxter and the NUJ accused the force of unjustifiably preventing journalists from doing their jobs by reacting heavy handedly to uncorroborated allegations by Trump's staff that the film-makers had been on their property without permission.

In January, the charges against both men were dropped. After a long inquiry, an internal complaints investigator at Grampian police has admitted to Baxter that the two officers who arrested him "could have interacted more effectively with both you and your colleague".

Chief Inspector Martin Mackay said that watching Baxter's film, "I can understand why a member of the public could have perceived the police actions within the documentary as being rash and confrontational and this has caused me some concern".

He also explicitly apologised for a police press release in May 2011 claiming that their arrests had been "corroborated by independent witnesses". This was wrong and misleading, since the witnesses were in fact Trump employees. "I can understand why this would cause you offence and a feeling of grievance," Mackay added.

He said it was not possible to reach a stronger conclusion about their arrest because Baxter had not allowed him to see the unbroadcast unedited footage of the incident. Mackay also said the force had acted properly and professionally throughout, and were justified in arresting him.

Baxter said he remains very angry about the incident, claiming that Mackay's inquiry had "made a mockery" of the claims to uphold professional standards. He said the arrest of working journalists as they conducted interviews was unprecedented in Britain.

"If these were normal 'force procedures' as the officers maintained, then journalists would be jailed in their hundreds every month in Scotland," Baxter said, in a response to Mackay's findings.

It emerged several months ago that Grampian police came under intense pressure from Trump to be more robust investigating his complaints about protests against his resort and step up their patrols of his estate. The police rejected his demands, in effect rebuking the billionaire property developer for misunderstanding their neutral role.